The invention described herein was made in the course of, or under, a contract with the U.S. ATOMIC ENERGY COMMISSION. It relates to electrically pumped gas lasers and more particularly to electrically pumped high-energy gas laser amplifiers in which gas breakdown of the lasing medium is substantially obviated.
When gases are irradiated by high intensity laser pulses, free electrons located within the beam path may be elevated to sufficient energies to collisionally ionize other gas molecules and/or atoms, resulting in a rapid growth of the free electron number density. The coefficient for absorption of photons by free electrons increases as the electron density increases, hence the electron cascade results in an abrupt attenuation of the laser beam. This attenuation of the laser pulse and resultant heating of the gas is defined as gas breakdown. It is also known in the literature as optical breakdown. Unfortunately, breakdown may occur in any gas, including those which constitute lasing mediums, if the laser pulse is sufficiently intense. It is thus readily apparent that gas breakdown constitutes a very substantial problem that must be overcome in the design of high-energy, gas laser amplifiers.
The literature discloses that the breakdown thresholds for many gases by light of various wavelengths have been determined experimentally as well as theoretically. The breakdown threshold is a function of gas constituents, pressure, temperature, laser frequency, and pulse length. The most recent data on CO.sub.2 laser mixtures is provided by Rockwood et al., 9 IEEE J.Q.E. 154 (1973). These data may be roughly summarized by ##EQU1## FOR PULSE LENGTHS T.sub.P &lt;&lt; 1 NSEC WHERE .lambda. IS THE LASER WAVELENGTH IN .mu.M AND P IS THE GAS PRESSURE IN ATMOSPHERES. This dependence switches to ##EQU2## FOR T.sub.P &gt;&gt; 1 NSEC.
In all instances, and regardless of the gas, the threshold is set by a certain energy or power per unit area. Thus to pass more energy without inducing breakdown, the area over which the energy is distributed must be increased. In the case of a conventional electrically pumped rectangular amplifier, this presents a difficult problem in that the cross-sectional area is fixed and independent of length. It is, of course, possible to alter the cross-sectional area of the pumped lasing medium by increasing the distance between the electrodes. This approach, however, rapidly results in such an increase in the required voltage across the electrodes that a point is quickly reached at which it is not feasible to achieve any higher energy output.